1. Field of the Invention
This invention relates to antithrombogenic elastomers, molded products made thereof, and a process for producing such elastomers.
2. Description of the Prior Art
Antithrombogenic elastomers exhibit their salient features when they are used at a site whereat they are brought into contact with blood. They are required for the manufacture of medical instruments which are brought into direct contact with blood, particularly artificial hearts, and other artificial organs. They are used for making, for example, blood vessel catheters, monitoring tubes, blood bags, externalcorporeal circulation circuits, such as artificial kidneys and mechanical hearts and lungs, A-V shunts, blood bypass tubes, artificial hearts, auxiliary artificial hearts, blood pumps, balloon pumps, etc. These applications essentially require the use of elastomers which are superior in antithrombogenic properties, mechanical properties (such as, for example, mechanical strength, elasticity and durability) and moldability.
Specific examples of antithrombogenic elastomers which are known in the art, include general purpose high molecular materials, such as soft vinyl chloride, polyurethane and silicone rubber; segmented polyurethane, such as Biomer of Ethicon, U.S.A.; heparinized polyurethane elastomer, which is disclosed in Japanese Pat. No. 13729/80; and a copolymer of polysiloxane and polyurethane bonded directly to each other by nitrogen and silicon, which is disclosed in U.S. Pat. No. 3,562,352.
None of these prior art compositions are entirely satisfactory for the use desired. For example, the general purpose high molecular materials are somewhat deficient in antithrombogenic properties. The segmented polyurethane is inferior in antithrombogenic properties, although it has high degree of mechanical strength. The heparinized polyurethane elastomer is very low in antithrombogenic properties after it has released heparin. Also, since heparin is physiologically active, this elastomer involves complex problems in molding and sterilization, and is thus costly to use. The copolymer disclosed in U.S. Pat. No. 3,562,352 (which is known under the tradename AVCOTHANE) is superior to any other known material in antithrombogenic properties, and is thus often used clinically. However, this copolymer is inferior in mechanical properties, when compared, for example, to segmented polyurethane. Moreover, its antithrombogenic properties, depend largely on the molding conditions, and are not always satisfactory. The copolymer is manufactured by a process which comprises mixing solutions of polyurethane and polysiloxane having reactive terminal groups, and reacting them when a molded product is formed therefrom. The reaction is shown by the following reaction equation: ##STR1## wherein R.sub.1 and R.sub.2 each stand for a polyurethane chain segment, R.sub.3 stands for a polysiloxane chain, and X stands for a substituent bonded to a silicon atom. As is obvious from this equation, it is in substance a graft copolymer. Also, since polysiloxane has at least two reactive terminal groups, it is a thermosetting resin containing polysiloxane as a cross linking agent. This resin is therefore, moldable by only a limited number of methods, such as by coating and dipping. This limitation to the molding method necessitates storage of the resin in the form of a solution. Disadvantageously, if the polyfunctional terminal groups of polyurethane and polysiloxane react with each other during storage, the solution will become highly viscous or gelatinous, and would be unusable.
Thus, there is in the art a large deficiency. To now, there does not exist any antithrombogenic elastomer which has the combined properties of excellent antithrombogenic properties, excellent mechanical properties, and molding properties which enable a wide range of molding methods.